Referring to FIG. 1, a crystalline silicon nanoparticle Si naturally includes, on the surface, a layer of oxide SiO2, the thickness ΔR of which is about one tenth of the overall diameter D of the nanoparticle. By quantum confinement, such nanoparticles, having a crystalline silicon core of the order of 10 nanometers or less in size, exhibit photoluminescence properties. The emission wavelength of the photons through the photoluminescence effect is shorter the smaller the size of the crystalline core of the nanoparticles (from 10 nm for emission in the red/infrared to 3 nm for emission in the red/yellow). Moreover, it is also possible to modify the emission wavelengths, especially by modifying the composition of the nanoparticles. For example, it is pointed out that carbon-containing (Si—C) nanoparticles and/or germanium-containing (Si—Ge) nanoparticles and/or nitrogen-containing (Si—N) nanoparticles may also have photoluminescence properties.
Thus, silicon, in the form of a powder of crystalline nanograins, is photoluminescent. This property, observable at room temperature and in the visible range (from green to red, depending on the grain size), may open the way for applications in very diverse fields, such as photonics (silicon lasers), biology (labels or tracers), counterfeit detection (optical barcode) cosmetics, etc.
The photoluminescence ascribed to the phenomenon of quantum confinement is observed when the structuring of the silicon is reduced to a nanoscale (size less than 10 nm) and the observed color varies according to the size of the nanocrystals. It will therefore be understood that there is a growing interest in producing silicon nanocrystals of as small a size as possible. However, the development of devices based on such nanoparticles has been retarded by the lack of availability of said nanoparticles.
Several approaches have been developed for obtaining these materials. Several techniques may for example be mentioned which result in thin Si-containing films, such as the implantation of Si into an SiO2 matrix followed by annealing, which has the effect of precipitating silicon nanocrystals. PECVD (plasma-enhanced chemical vapor deposition) may also be mentioned. Furthermore, approaches in liquid phase, such as what is called “reverse micelle” synthesis, or else supercritical solutions, are often lengthy to implement and give low yields.
As regards physical synthesis processes for obtaining free nanoparticles, the following may be mentioned: laser ablation; thermal pyrolysis in a fluidized reactor, plasma-induced synthesis; and laser-induced pyrolysis (using a sufficiently powerful laser such as for example a CO2 laser). Only the latter two techniques have given useful results as regards production and quality of the nanoparticles obtained.
According to a first technique, silicon nanoparticles in the size range from 2 to 8 nm have been obtained in a low-pressure (186 to 1860 Pa) argon/silane radiofrequency plasma by the process described in: “High yield plasma synthesis of luminescent silicon nanocrystals” by L. Mangolini, E. Thimsen and U. Korsthagen, Nanoletters, Vol. 5-4, pp. 655-659, (2005). Silane flow rates in the 0.4 to 2.4 sccm (standard cubic centimeters per minute) range allow production rates ranging from 14 to 52 mg per hour to be achieved.
The nanoparticles have quite a narrow size distribution (for example 5.72 nm with a standard deviation of 0.68 nm). After what is called a “passivation” treatment aimed at making the surface of the nanoparticles inert, they exhibit photoluminescence with a peak wavelength between 700 and 840 nm, depending on the nanoparticles.
A second technique is the laser pyrolysis of silane, especially using a CO2-based pulsed laser, according to the process described in: “Photoluminescence properties of silicon nanocrystals as a function of their size” by G. Ledoux, O. Guillois, D. Porterat, C. Reynaud, F. Huisken, B. Kohn and V. Paillard, Phys. Rev. B, Vol. 62(23), pp. 15942-51 (2000). It will be recalled that this technique consists in exposing a stream of silane to a laser beam so as to raise its temperature up to the point of obtaining a pyrolysis flame and to collect silicon nanoparticles that are formed in the flame. The nanoparticles obtained lie within a size range between 2.8 and 4.8 nm and emit intense photoluminescence in the 610-900 nm wavelength range after simple passivation in air. However, the very small quantities obtained (only a few milligrams) seem incompatible with industrial applications.
More recently, it has been shown that it is possible to produce silicon nanoparticles in the 5-20 nm size range by laser pyrolysis of silane at a pressure of 54×103 Pa, using a continuous, but focused, low-power (60 W) laser (with a focal spot diameter of 2 mm), in the document: “Process for Preparing Macroscopic Quantities of Brightly Photoluminescent Silicon Nanoparticles with Emission Spanning the Visible Spectrum” by X. Li, Y. He, S. Talukdar, M. Swihart, Langmuir 19, pp. 8490-8496 (2003). The production rate is in the 20-200 mg/hour range. However, after the synthesis, a second step (here a chemical treatment by an HF/HNO3 compound) is necessary to make the particles photoluminescent. The photoluminescence can then be observed in the 500-800 nm wavelength range.
In that same document it is indicated that production rates would be higher using a higher-power laser and by increasing the size of the laser/precursor overlap area. However, no details are given about the effect of these parameters on the particle sizes. Nevertheless, an increase in crystal size is expected since an increase in the laser power would a priori lead to an increase in the flame temperature and therefore, in general, to better crystallization. Furthermore, increasing the overlap area would itself also lead to an increase in the interaction time and therefore in the size of the crystals obtained.
Silicon nanoparticles with a size possibly down to around 12 nm have been obtained by laser pyrolysis, using a high-power continuous laser, as described in: “Laser-grown silicon nanoparticles and photoluminescence properties” by N. Herlin-Boime, K. Jursikova, E. Trave, E. Borsella, O. Guillois, J. Vicens and C. Reynaud, Proceedings, MRS Spring Meeting, Symposium M, San Francisco, USA (2004). The production rates obtained are several grams per hour. Because of their size, these nanoparticles do not exhibit photoluminescence. Only by a heat treatment step in air (so as to force oxidation) could the size of the silicon core be reduced to below 10 nm and therefore the photoluminescence effect obtained.
Thus, as regards laser pyrolysis, there is presently no solution making it possible to obtain, in weighable amount and in a single step, silicon nanoparticles having a size of 10 nm or less and exhibiting photoluminescence properties, for example after simple passivation in air.
The aim of the present invention is to improve the situation.